The present invention relates to high-voltage stator coils and more particularly to methods and apparatuses for inhibiting electrical discharge between vent tubes and strands in inner-cooled stator coils. Although the following discussion focuses on stator coils for turbogenerators, the present invention is applicable to other dynamoelectric machines, including electric motors.
Conventional turbogenerators have a rotor and a stator. The rotor is wound with field windings, which are disposed in slots in the body of the rotor. The stator is wound with stator coils, which are disposed in slots in the body of the stator. When the rotor is rotated by an external source of mechanical energy, such as a steam turbine or a gas turbine, and an excitation current is provided to the field windings, electrical energy is induced in the stator coils.
Stator coils are generally constructed from a plurality of individual conductors referred to as strands. The strands are stacked together to form a larger conductor (or coil) capable of carrying high voltages and currents. In many stator coils, the strands are twisted into a weaved pattern rather than simply being stacked one on top of another. This weaving technique is known as Roebelling. elling helps prevent the inner strands of a stator coil, which are closest to the rotor, from carrying more current (and generating more heat) than the outer strands, which are further from the rotor. elling helps ensure that each strand carries a similar amount of current and generates a similar amount of heat.
Some stator coils include integral vent tubes to help cool the strands. These types of stator coils are referred to as inner-cooled coils. In inner-cooled coils, a plurality of vent tubes are generally stacked on top of one another and sandwiched between two or more stacks of strands. A cooling gas like hydrogen or air is then pumped through the vent tubes to help transfer heat away from the strands.
There are a number of challenges associated with manufacturing inner-cooled stator coils. For example, after a stack of strands has been elled, the top and bottom surface of the stack is no longer smooth. The surfaces have significant irregularities or indentations caused by the elling of the strands. These irregularities make it difficult to apply the outer layer of insulation, referred to as ground-wall insulation.
Another challenge involves the fact that an extremely large voltage differential can appear between the strands and the vent tubes in a stator coil while a generator is operating. If this voltage differential exceeds the dielectric strength of the insulation between the strands and the vent tubes, an electrical short will occur between the copper strands and the vent tubes, which can lead to circulating currents in the vent tubes and catastrophic damage to the stator coil.
In an effort to inhibit electrical shorts between stands and vent tubes, U.S. Pat. No. 6,624,547 to Emery, which is incorporated by reference herein in its entirety, discloses reducing the potential difference between copper strands and vent tubes by introducing a compact voltage grading means between the copper strands and the vent tubes. This grading means is formed from conductive strips that are positioned between a stack of strands and a stack of vent tubes. The conductive strips are fixed in place with multiple layers of insulating tape and provide electrical grading between the strands and vent tubes. Despite the significant advancement provided by this approach, there remains a continued need for improvements in stator coil configurations than provide increased protection against electrical shorts, while also reducing the complexity and costs associated with manufacturing stator coils.